The OS would be helpful.
If you can still boot the PC, boot into 'Safe Mode' and run AVG from there. Before you do so turn off 'System Restore' to negate any reinfection from there.
If you can`t boot, go to click here and download the rescue CD and make a copy. Boot the PC from this and use it to clean the malware.
Hopefully you will clear the problem and be able to boot. If not, you could try a 'Repair'. If XP click here or click here If Vista click here If W7 click here